Method for fabricating leadless solder for IC packaging

ABSTRACT

A novel method for fabricating leadless solder for IC packaging includes the following steps: A. to prepare a mixture containing Zn, 7-11 wt %; Co, 0.1-0.5 wt %; and Sn for balance; or a mixture containing Zn, 7-11 wt %; Co, 0.1-0.5 wt %; P, 0.005-0.05 wt % and Sn for balance; or a mixture containing Zn, 7-11 wt %; Al, 0.1-0.5 wt %; and Sn for balance; a mixture containing Zn, 7-11 wt %; Al, 0.1-0.5 wt %; P, 0.005-0.05 wt % and Sn for balance; or a mixture containing In, 10-15 wt %; Co, 0.1-0.5 wt % and Sn for balance; B. to place the mixture in a high frequency furnace to be melted together at a temperature lower than 350° C., then take it out of the furnace for a quick freezing and cast it into an ingot in an adequate size; C. to treat the ingot with a homogenization processing at 180-240° C. for 30-240 minutes; D. to draw the homogenized ingot into the form of leadless solder filament or granule.

FIELD OF THE INVENTION

[0001] The present invention relates to a novel technique for fabricating IC packaging material, in particular the leadless solder to ensure high reliability of IC packaging and rigorous compliance with environment protection requirements.

DESCRIPTION OF THE PRIOR ART

[0002] The semiconductor technology advances in a rapid pace and the energy reservation in the electronic product has brought new creation one after another. In response to the wide recognition of green ecology all over the world, the new generation of semiconductor production technology and material is continuously under strong plea to include the green world perception. In the past decade, the lead pollution generated from the scrapped electronic products has drawn great attention in the industry field Some nations such as EC have enacted regulations to ban the application of lead-contained product for the sake of environment protection.

[0003] The lead-contained solder has been traditionally used in IC packaging material. The main reason for its application lies on the fact that the electronic elements can only withstand a temperature ceiling about 220° C., so the heat the solder has to withstand should be less than 220° C. Traditionally, it was attempted to produce a mixture of lead and zinc in a proper ratio which has a heat resistance lower than 220° C. As shown in FIG. 1, a mixture of 63% zinc and 37% lead is the most optimal combination having a melting temperature close to 183° C., lower than the temperature ceiling 220° C. for electronic elements to withstand. Therefore, lead-contained solder is commonly adopted as IC packaging material.

[0004] As application of lead-contained solder is banned, up to now, many efforts to develop leadless solder substitutions fail to accomplish the goal that the melting temperature of the new alloy solder is less than 183° C. To collaborate such high characteristic requirements derived from leadless solder, heat resistance of electronic elements has been improved to reach as high as 250° C., permitting the application of leadless solder with a melting temperature of 210° C. (40° C. lower than the heat resistance electronic elements can withstand.)

[0005] Nowadays, the solder substitution most IC manufacturers prefer is a mixture of Sn—Bi, which features high humidity, easiness to operate and no generation of fine filament which is the culprit of short circuit in finished products. However, during the soldering processing, it is often found that bismuth is easily susceptible to settlement to produce a life-off and the bismuth-contained solder is easy to peel off. These are key reasons the IC manufacturers are unwilling to use bismuth-contained solder. To develop a substitution solder with long efficiency, high reliability and complete satisfaction to requirements the leadless solder embraces is the goal all studies and researches of the industry are aimed at.

[0006] For this reason, the inventor has worked hard for years in developing leadless solder and come up with a novel method for mixing the leadless solder for IC packaging to satisfy the IC industrial need.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows the melting points of Zn—Pb solder at varying combination.

[0008]FIG. 2 is a block diagram showing the process flow of fabricating leadless solder of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for mixing leadless solder for IC packaging according to this invention is explained in great details as follows:

[0010]FIG. 2 shows the proper procedures to fabricate the leadless solder of the invention:

[0011] A. To prepare five metal alloys, a mixture containing Zn, 7-11 wt %; Co, 0.1-0.5 wt %; and Zn for balance; a mixture containing Zn, 7-11 wt %; Co, 0.1-0.5 wt %; P, 0.005-0.05 wt % and Zn for balance; or a mixture containing Zn, 7-11 wt %; Al, 0.1-0.5 wt %; and Zn for balance; or a mixture containing Zn, 7-11 wt %; Al, 0.1-0.5 wt %; P, 0.005-0.05 wt % and Zn for balance; or a mixture containing In, 10-15 wt %; Co, 0.1-0.5 wt % and Zn for balance.

[0012] B. To place the mixture in a high frequency furnace to be melted together at a temperature lower than 350° C.; then take it out of the furnace for a quick freezing and cast it into an ingot in an adequate size.

[0013] C. To treat the ingot with a homogenization processing at 180-240° C. for 30-240 minutes. It presents a smooth surface without dents and pits.

[0014] D. To drawn the homogenized ingot into the form of leadless solder filament or granule.

[0015] As described above, these five mixtures of leadless solder differ in the ingredients, and each inheres varying characteristics, but the critical criteria to be considered are the melting temperature which is confined to less than 210° C. The criteria these five leadless solders embrace as experiments indicate are illustrated in the following.

[0016] (I) For mixture containing Zn 7-11 wt %, Co 0.1-0.5 wt %, and Zn for balance, the solidification temperature is about 199° C., liquidation temperature, 200° C.; melting temperature, between 199° C.-200° C., tensile strength, 6.2 kgf/mm²; and elongation, 68%.

[0017] (II) For mixture containing Zn 7-11 wt %, Co 0.1-0.5 wt %, P 0.005-0.05 wt % and Zn for balance, in which a meager amount of phosphor is added in the (I) mixture, its tensile strength and elongation are therefore reduced.

[0018] The melting temperature ranges between 199° C.-200° C., tensile strength, 5.8 kgf/mm² and elongation, 60%.

[0019] (III) For mixture containing Zn 7-11 wt %, Al 0.1-0.5 wt %, and Zn for balance, in which cobalt is replaced by aluminum for similar nature and low cost. The solidification temperature is about 196° C., liquidation temperature, 197° C. The melting temperature ranges between 199° C.-200° C., tensile strength, 6.2 kgf/mm² and elongation, 67%.

[0020] (IV) For mixture containing Zn 7-11 wt %, Al 0.1-0.5 wt %, P 0.005-0.05 wt % and Zn for balance, in which a meager amount of phosphor is added, and its tensile strength and elongation are therefore reduced. The melting temperature ranges between 196° C.-197° C., very similar to the mixture (III), the tensile strength, 5.8 kgf/mm²; and elongation, 59%.

[0021] (V) For mixture containing In 10-15 wt %, Co 0.1-0.5 wt %, and Zn for balance, in which indium takes the place of zinc. Because of the existence of indium, the melting temperature is slightly low, the solidification temperature is 183° C., the liquidation temperature, 195° C. and the melting temperature ranges between 183° C.-195° C. The tensile strength, 5.7 kgf/mm²; and elongation is as low as 33%.

[0022] It is apparent that with proper ratio of these five mixtures, leadless solder can be fabricated, which satisfies the requirements environment protection authorities set out.

[0023] It is therefore understood that the leadless solder for IC packaging fabricated in accordance with the method of the invention cherishes the advantages of high oxidative prevention. Besides, the soldering temperature is as low as 250° C., liquidation temperature and solidification temperature are very similar, the soldering temperature equals to Zn 63% Pb37% solder, the tensile strength is as high as 5 kgfmm², elongation is above 10%, solderability is outstanding, which conforms to the requirements IC packaging material sets forth. The application of leadless solder lowers repair rate and reduces production cost, thus benefits IC manufacturing industry.

[0024] The application of leadless solder fabricated in accordance with this invention assures high reliability in IC packaging and strict compliance with the environment protection. 

What is claimed is:
 1. A method for fabricating leadless solder for IC packaging comprising at least following steps: A. to prepare five metal alloys, a mixture containing Zn, 7-11 wt %, Co, 0.1-0.5 wt %, and Zn for balance, a mixture containing Zn, 7-11 wt %, Co, 0.1-0.5 wt %, P, 0.005-0.05 wt % and Zn for balance, or a mixture containing Zn, 7-11 wt %; Al, 0.1-0.5 wt %, and Zn for balance, or a mixture containing Zn, 7-11 wt %, Al, 0.1-0.5 wt %, P, 0.005-0.05 wt % and Zn for balance, or a mixture containing In, 10-15 wt %, Co, 0.1-0.5 wt %, wt % and Zn for balance; B. to utilize a high frequency furnace to melt said mixture together at a temperature lower than 350° C., then take it out of the furnace for a quick freezing and cast it into an ingot in an adequate size; C. to treat said ingot with a homogenization processing at 180-240° C. for 30-240 minutes to present a smooth surface without dents and pits; and D. to drawn said homogenized ingot into a form of leadless solder filament or granule.
 2. The finished leadless solder as fabricated in accordance with the steps A through D of claim 1 has the liquidation temperature and solidification temperature ranging from 183° C. to 200° C., making the soldering temperature less than 250° C. to assure highly reliable packaging in IC production.
 3. The finished leadless solder as fabricated in accordance with the steps A through D of claim 1 has the tensile strength between 5.7-6.2 kgf/mm², greater than 5 kgf/mm², elongation between 33% and 68%, greater than 10% to ensure better solderability. 